In a liquid crystal display device, a classification based on an operating mode for liquid crystal molecules includes a phase change (PC) mode, a twisted nematic (TN) mode, a super twisted nematic (STN) mode, an electrically controlled birefringence (ECB) mode, an optically compensated bend (OCB) mode, an in-plane switching (IPS) mode, a vertical alignment (VA) mode, a fringe field switching (FFS) mode and a field-induced photo-reactive alignment (FPA) mode. A classification based on a driving mode in the device includes a passive matrix (PM) and an active matrix (AM). The PM is classified into static, multiplex and so forth, and the AM is classified into a thin film transistor (TFT), a metal insulator metal (MIM) and so forth. The TFT is further classified into amorphous silicon and polycrystal silicon. The latter is classified into a high temperature type and a low temperature type based on a production process. A classification based on a light source includes a reflective type utilizing natural light, a transmissive type utilizing backlight and a transflective type utilizing both the natural light and the backlight.
The liquid crystal display device includes a liquid crystal composition having a nematic phase. The composition has suitable characteristics. An AM device having good characteristics can be obtained by improving characteristics of the composition. Table 1 below summarizes a relationship in two characteristics. The characteristics of the composition will be further described based on a commercially available AM device. A temperature range of the nematic phase relates to a temperature range in which the device can be used. A preferred maximum temperature of the nematic phase is about 70° C. or higher, and a preferred minimum temperature of the nematic phase is about −10° C. or lower. Viscosity of the composition relates to a response time in the device. A short response time is preferred for displaying moving images on the device. A shorter response time even by one millisecond is desirable. Accordingly, small viscosity in the composition is preferred. Small viscosity at low temperature is further preferred.
TABLE 1Characteristics of composition and AM deviceNo.Characteristics of compositionCharacteristics of AM device1Wide temperature range of aWide usable temperature rangenematic phase2Small viscosity1)Short response time3Suitable optical anisotropyLarge contrast ratio4Large positive or negativeLow threshold voltage anddielectric anisotropysmall electric power consumptionLarge contrast ratio5Large specific resistanceLarge voltage holding ratio andlarge contrast ratio6High stability to ultravioletLong service lifelight and heat7Large elastic constantLarge contrast ratio and shortresponse time1)A composition can be injected into a liquid crystal display device in a short time.
Optical anisotropy of the composition relates to a contrast ratio in the device. According to a mode of the device, large optical anisotropy or small optical anisotropy, more specifically, suitable optical anisotropy is required. A product (Δn×d) of the optical anisotropy (An) of the composition and a cell gap (d) in the device is designed so as to maximize the contrast ratio. A suitable value of the product depends on a type of the operating mode. In a device having a mode such as a TN mode, the value is about 0.45 micrometer. In a device having the VA mode, the value is in the range of about 0.30 micrometer to about 0.40 micrometer, and in a device having the IPS mode or the FFS mode, the value is in the range of about 0.20 micrometer to about 0.30 micrometer. In the above case, a composition having large optical anisotropy is preferred for a device having a small cell gap. Large dielectric anisotropy in the composition contributes to low threshold voltage, small electric power consumption and a large contrast ratio in the device. Accordingly, large positive or negative dielectric anisotropy is preferred. Large specific resistance in the composition contributes to a large voltage holding ratio and the large contrast ratio in the device. Accordingly, a composition having large specific resistance at room temperature and also at a temperature close to the maximum temperature of the nematic phase in an initial stage is preferred. The composition having large specific resistance at room temperature and also at a temperature close to the maximum temperature of the nematic phase even after the device has been used for a long period of time is preferred. Stability of the composition to ultraviolet light and heat relates to a service life of the device. In the case where the stability is high, the device has a long service life. Such characteristics are preferred for an AM device use in a liquid crystal projector, a liquid crystal television and so forth.
In a liquid crystal display device having a polymer sustained alignment (PSA) mode, a liquid crystal composition containing a polymer is used. First, a composition to which a small amount of a polymerizable compound is added is injected into the device. Next, the composition is irradiated with ultraviolet light while voltage is applied between substrates of the device. The polymerizable compound is polymerized to form a network structure of the polymer in the composition. In the composition, alignment of liquid crystal molecules can be controlled by the polymer, and therefore the response time in the device is shortened and also image persistence is improved. Such an effect of the polymer can be expected for a device having the mode such as the TN mode, the ECB mode, the OCB mode, the IPS mode, the VA mode, the FFS mode and the FPA mode.
In a general-purpose liquid crystal display device, vertical alignment of liquid crystal molecules is achieved by a polyimide alignment film. On the other hand, as a liquid crystal display device having no alignment film, a mode of adding a polar compound to a liquid crystal composition and aligning liquid crystal molecules has been proposed. First, a composition to which a small amount of the polar compound and a small amount of the polymerizable compound are added is injected into the device. Here, the liquid crystal molecules are aligned by action of the polar compound. Next, the composition is irradiated with ultraviolet light while voltage is applied between substrates of the device. Here, the polymerizable compound is polymerized to stabilize the alignment of liquid crustal molecules. In the composition, the alignment of the liquid crystal molecules can be controlled by the polar compound and the polymer, and therefore the response time in the device is shortened, and image persistence is improved. Further, in the device having no alignment film, a step of forming the alignment film is unnecessary. The device has no alignment film, and therefore reduction in electric resistance of the device by interaction between the alignment film and the composition is not caused. Such an effect caused by a combination of the polar compound and the polymer can be expected for the device having the mode such as the TN mode, the ECB mode, the OCB mode, the IPS mode, the VA mode, the FFS mode and the FPA mode.
In the liquid crystal display device having no alignment film, various compounds each having a —OH group at a terminal have been so far prepared as a compound in which liquid crystal molecules can be vertically aligned. Patent literature No. 1 describes biphenyl compound (S-1) having a —OH group at a terminal. However, in the compound, capability of vertically aligning liquid crystal molecules is high, but the voltage holding ratio is not sufficiently large when the compound is used in the liquid crystal display device.
